This invention relates to an oxygen regulator for supplying an aviator with a breathable fluid in response to an inhalation demand. At altitudes above 40,000 feet ambient atmospheric pressure becomes so low that aviators can suffer harmful effects such as black-outs. Therefore, it is essential that at least a minimum amount of oxygen supplements the breathable air supplied to the aviator. To add such oxygen to the air supply system is common practice to include a demand regulator between a source of oxygen and the aviator breathing mask. Most demand oxygen regulators include an aneroid responsive valve which proportions the amount of oxygen and air supplied to the mixing chamber for distribution to the recipient. In such oxygen regulators, of which U.S. Pat. No. 3,496,954 is typical, a fixed proportion of ambient air and oxygen are supplied to a recipient below a predetermined elevation and when the aircraft goes above the predetermined elevation, the amount of air is proportionally reduce and the amount of oxygen is proportionally increased. Above 40,000 feet it is normal procedure that oxygen alone be supplied to the aviator. Unfortunately with fixed proportioning the breathable air does not meet every aviator's inhalation demand.